1. Field of the Invention
This invention relates to a digital servo-control system suitable for driving a machine with a number of drive shafts such as a robot or a machine tool.
2. Description of the Prior Art
In order to drive a machine with a number of drive shafts such as a robot or a machine tool, a plurality of motors must be controlled simultaneously. Each motor is driven through a servomechanism. In controlling axial positions through a servo system, a microprocessor is typically used as an axial position control means. This technique is described in, for example, "Assembly Robot and Its Technological Problems" by Koichi Sugimoto et al in the Hitachi Review, Vol. 64, No. 12, December 1982, pages 5 to 10.
Incidentally, a number of one-axis servo systems have hitherto been combined to constitute a multi-axis servo system. In recent years, however, demands for high performance and cost reduction have arisen and the aforementioned arrangement has not always been evaluated to be the most profitable arrangement. Accordingly, it has been considered to control a number of shafts by using a multi-axis position control means employed in common to a plurality of servo systems. An example of this type of arrangement will be described briefly.
The multi-axis position control means (microcomputer) receives a position command value for each axis and a position detection value detected for each axis and executes an arithmetic operation to determine a speed command value for each axis. The speed command value for each axis thus determined by the multi-axis position control means is applied to a speed control circuit provided for each axis. The speed control circuit of each axis compares a speed detection value from a speed detector provided for each axis with the speed command value to produce a current command value (torque command value) for each axis. The current command value is applied to a current control circuit. The current control circuit of each axis compares the current command value with a current detection value for a motor of each axis and controls a power converter which supplies electric power to the motor. Thus, position control is executed while controlling current and speed under the application of a voltage to the motor.
The digital servo control system for effecting multi-axis position control described above in brief, however, faces the following problems.
Initially, DC motors and others types of motors such as an induction motors or synchronous motors are used as servomotors at present and hence in multi-axis position controlling, it is sometimes desired to change the type of motor. The type of speed control circuit is usually determined in accordance with the type of motor used. Accordingly, when the motor is exchanged, a servo drive system including the speed control circuit, current control circuit and power converter must also be exchanged. This exchange of the servo drive system is unacceptable from the standpoint of economy. In addition, the speed control circuit is operable in digitized configuration but it is provided for each axis, thereby unduly raising cost.
In the second place, the provision of the speed detector and position detector for each axis also raises cost. Further, the employment of the speed detector is contrary to a recent technical trend of effecting speed detection with a position signal supplied from the position detector.